1. Field of the Invention
The present invention relates to a DC-AC converter circuit for use in a discharge-lamp lighting circuit, which is made small in size and low in manufacturing cost.
2. Description of the Related Art
There is a known arrangement of component parts including a DC-DC converter circuit, a DC-AC converter circuit and a starter circuit to constitute a discharge-lamp lighting circuit such as a circuit for lighting a metal halide lamp and so forth.
The DC-AC converter circuit positioned at the rear stage of the DC-DC converter circuit is used for supplying the AC voltage converted from the DC input voltage to a discharge lamp and has a bridge type circuit using a plurality of semiconductor switching elements and a driving circuit. More specifically, the driving circuit made of a driving IC (Integrated Circuit) supplies control signals to the respective semiconductor switching elements. Alternating operation of the bridge circuit is performed by the control signals that define the on-off conditions of the switching elements, so that the direct current is converted to the alternating current.
In a typical conventional circuit of the sort mentioned above, there are develop cost and packaging-area problems when the driving IC is used as the driving circuit for the switching elements forming the bridge circuit. In other words, the use of an expensive driving IC causes an increase in the whole circuit cost and moreover the packaging area of such a driving IC constitutes an obstacle to reducing its size.
Although there may be considered a method of attempting to reduce the cost by assembling a circuit having the same function as that of the driving IC from discrete parts, there are problems with the conventional arrangement in view of protecting the withstand voltage of the element and increasing the switching speed as follows.
To drive the switching element, it may increase the cost further due to the necessity of using a high-withstand-voltage semiconductor element instead. The use of a high with stand voltage (approximately at 600V) element is needed in case where a FET (Field Effect Transistor) is employed, for example, as a single control element for controlling the on-off conditions of the switching element positioned on the high voltage side out of the arms constituting the bridge circuit. Otherwise, measures are needed to be taken to protect the element from being injured or damaged by a large current flowing from the bridge into the semiconductor element. The cost will also increases by taking such measures.
In case where the switching speed of the semiconductor element for driving the switching element is low, there is the possibility of impeding the operation of the bridge and this results in the necessity of using a high-speed element, which causes an increase in the cost.
An object of the invention is to provide a discharge-lamp lighting circuit in which a circuit for driving a switching element forming a DC-AC converter circuit is reduced in not only size but also cost.
A discharge-lamp lighting circuit comprising a DC-DC converter circuit for converting input voltage from a DC power supply to a desired DC voltage and a DC-AC converter circuit which is positioned at the rear stage of the DC-DC converter circuit and used for converting the DC voltage to AC voltage according to the invention is constituted of the following.
First, the DC-AC converter circuit is formed in accordance with a bridge type circuit arrangement including a plurality of switching elements and is provided with a driving circuit using a plurality of bipolar transistors for controlling the on-off conditions of the switching elements positioned on the high voltage side out of the switching elements forming a bridge; and the plurality of bipolar transistors are connected in series and the on-off conditions of the switching elements are controlled by the transistors positioned on the low voltage side, so that the withstand voltage of each transistor is lowered.
The bipolar transistors for controlling the on-off conditions of the switching element positioned on the high voltage side out of the switching elements forming the bridge are employed and the resistance element for limiting the current flowing through the bipolar transistor from the node between the switching elements forming the arms of the bridge is connected to the proper bipolar transistor, whereby the resistance element is prevented from being damaged by a large current.
A control circuit for controlling the condition of the bipolar transistors so that the base current flowing through the proper transistor before the transistor is turned off after the transistor is turned on once is made smaller than the base current flowing through the transistor when the transistor is turned on, whereby the switching speed of the transistor is improved (which makes it unnecessary to use any high-speed transistor).
It is therefore made possible to lower withstand voltages of transistors, to protect elements by current limiting resistance elements and to increase switching speed, whereby a switching element forming a DC-AC converter circuit is reduced in not only size but also cost.